Image pickup apparatus and controlling method therefor

ABSTRACT

An image pickup apparatus that is capable of reducing variation in the color temperature of a subject in a taken image and capable of shooting a more suitable image. The image pickup apparatus can shoot with emitting a light emitting unit. A color temperature acquisition unit acquires information about a color temperature of a light illuminating a subject. An emission control unit controls emission of the light emitting unit. The emission control unit determines emission light amount of the light emitting unit based on a color temperature difference between a color temperature of an illumination light of the light emitting unit and a color temperature based on the information acquired by the color temperature information acquisition unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application. Ser. No. 12/966,435,filed Dec. 13, 2010 the entire disclosure of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image pickup apparatus that enablesshooting with emitting a light emitting unit, and a control methodtherefor.

2. Description of the Related Art

Conventionally, a camera has an automatic exposure control(AutoExposure) function to set an aperture, a shutter speed, andsensitivity appropriately according to brightness of a subject at thetime of shooting.

A camera emits a strobe when shooting a backlight scene, such as a scenewhere a person stands before the sun, with a background in properbrightness, or when shooting in a low illumination environment with ahigh shutter speed in order to reduce a camera shake.

A color temperature of the strobe light is determined by acharacteristic of a light-emitting device. A xenon tube has a colortemperature of about 5500 to 6000 K.

On the other hand, sunlight outdoors in daytime has a color temperatureof about 5500 K, so it is much the same as that of the strobe light bythe xenon tube. On the other hand, there are light sources with a lowcolor temperature of about 2500 K such as a filament lamp.

When shooting with strobe emission under such a light source of whichthe color temperature is largely different from that of the light of thestrobe mounted on a camera, a white balance cannot be kept due tomixture of the color temperatures that are different to each other,which may make a taken image be unnatural.

SUMMARY OF THF INVENTION

The present invention provides an image pickup apparatus and a controlmethod therefor that are capable of reducing variation in the colortemperature of a subject in a taken image and capable of shooting a moresuitable image.

Accordingly, a first aspect of the present invention provides an imagepickup apparatus that is possible to shoot with emitting a lightemitting unit comprising a color temperature acquisition unit configuredto acquire information about a color temperature of a light illuminatinga subject, and an emission control unit configured to control emissionof the light emitting unit, wherein the emission control unit determinesemission light amount of the light emitting unit based on a colortemperature difference between a color temperature of an illuminationlight of the light emitting unit and a color temperature based on theinformation acquired by the color temperature information acquisitionunit.

Accordingly, a second aspect of the present invention provides a controlmethod for an image pickup apparatus that is possible to shoot withemitting a light emitting unit, the method comprising a colortemperature acquisition step of acquiring information about a colortemperature of a light illuminating a subject, and an emission controlstep of controlling emission of the light emitting unit, whereinemission light amount of the light emitting unit is determined in theemission control step based on a color temperature difference between acolor temperature of an illumination light of the light emitting unitand a color temperature based on the information acquired in the colortemperature information acquisition step.

According to the present invention, the variation in the colortemperature of the subject in a taken image can be reduced, and a moresuitable image can be shot.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically showing a configuration of acamera according to an embodiment of the present invention.

FIG. 2A and FIG. 2B are views showing two examples of persons' facesthat are exposed to different extraneous lights.

FIG. 3A, FIG. 3B, and FIG. 3C are views showing combination examples ofexposure settings and strobe emissions.

FIG. 4 is a view showing areas used for acquisition of brightnessdistribution.

FIG. 5 is a view showing face information recognized by an imagerecognition circuit.

FIG. 6A and FIG. 6B are the views showing areas in a face acquired asinformation about a histogram.

FIG. 7A, FIG. 75, FIG. 7C, and FIG. 7D are histograms showing brightnessdistributions of the respective areas in the face shown in FIG. 6A andFIG. 6B.

FIG. 8 is a flowchart showing an exposure compensation value calculationprocedure.

FIG. 9 is a graph showing correlation between target brightnesscorresponding to a color temperature of an extraneous light and anunder-compensation step.

FIG. 10 is a graph showing correlation between target brightnesscorresponding to a face contrast level and an under-compensation step.

FIG. 11 is a graph showing an exposure under-compensation limit valuecorresponding to subject brightness.

FIG. 12 is a flowchart showing an entire procedure of a shootingoperation.

DESCRIPTION OF THE EMBODIMENTS

Hereafter, embodiments according to the present invention will bedescribed in detail with reference to the drawings.

FIG. 1 is a block diagram schematically showing a configuration of acamera according to the present embodiment. The camera 1 condenses anextraneous light by a lens 10. The camera 1 adjusts a focal point and afield angle by moving the lens 10 forward and backward along an opticalaxis by a lens driving circuit 42.

The camera 1 drives the lens 10 so as to change the optical axis in adirection to cancel hand shake by an image stabilization circuit 40based on a shake amount of the camera measured by a shake amountdetection circuit 44, and compensates the hand shake optically. Theshake amount detection circuit 44 is provided with a gyro sensor.Although the image stabilization is achieved by driving the lens 10 inthe configuration shown in FIG. 1, it can be also achieved by driving animage pickup device 16 such as a CCD or a CMOS.

Light amount passed through the lens 10 is adjusted by a diaphragm 14. Asystem control unit 60 controls the diaphragm 14 by transmittingdiaphragm control information to a diaphragm driving circuit 26.

The system control unit 60 controls the diaphragm 14 via the diaphragmdriving circuit 26 so as to stop down the diaphragm to drop the lightamount when a subject brightness is high, and so as to open thediaphragm to take much light when the subject brightness is low.

The system control unit 60 controls a mechanical shutter (it is alsocalled a “shutter”, simply) 12 by transmitting mechanical shuttercontrol information to a mechanical shutter drive circuit 28. The systemcontrol unit 60 determines open-to-close time for the shutter, andprovides instructions to the mechanical shutter drive circuit 28.

The light passed through the lens 10, the mechanical shutter 12, and thediaphragm 14 is received by the image pickup device 16. The systemcontrol unit 60 controls the image pickup device 16 by transmitting acontrol signal to a TG (Timing Generator) 24.

The TG 24 drives the image pickup device 16 according to the controlsignal received from the system control unit 60. The image pickup device16 periodically executes an exposure process and a data read-outprocess. These processes are executed on the basis of a driving signalfrom the TG 24.

The TG 24 controls exposure time of the image pickup device 16. Theexposure time can be controlled by outputting the driving signal fromthe TG 24 to the image pickup device 16 at an arbitrary timing so thatthe device opens the stored electric charge.

Image data read out from the image pickup device 16 is inputted into aCDS (Correlated Double Sampler) 18. The CDS 18 removes a noise componentof the image data inputted by correlation double sampling systems.

The image data processed by the CDS 18 is inputted into a PGA(Programmable Gain Amplifier) circuit 20, and the level of the imagedata is attenuated or amplified.

The system control unit 60 controls an amplifying ratio by transferringan amplifying level to the PGA 20. Usually, correct exposure of theimage pickup device 16 is obtained by setting the diaphragm 14 so thatthe exposure amount of the image pickup device 16 becomes appropriate,and by setting exposure time appropriately by the shutter etc. On theother hand, the PGA 20 attenuates or amplifies the image data to changethe exposure of the image data artificially. The artificial exposurechange provides a user with a function as a concept of sensitivity thatis one of the exposure conditions such as the shutter speed and anaperture at the time of shooting. The system control unit 60 controlsthe exposure by controlling these aperture, shutter speed, andsensitivity.

The image data outputted from the PGA 20 is converted into a digitalsignal from an analog signal by an A/D (Analog/Digital) converter 22.

The image data digitized by the A/D converter 22 is inputted into animage processing circuit 50 via a data bus 23. The image processingcircuit 50 comprises a plurality of blocks, and achieves variousfunctions.

The image pickup device 16 generally extracts a specific color componentfor every pixel through a color filter. The image data from A/Dconverter 22 has a data format that comprises a color componentcorresponding to a pixel of the image pickup device 16 and anarrangement of the color filter. This data format is not suitable for anautomatic exposure control (AE) that evaluates only a brightnesscomponent and controls exposure.

The image processing circuit 50 is provided with a function to eliminatethe color information from the image data and to extract only brightnessinformation. The image processing circuit 50 is also provided with afunction to extract the color information. The color information is usedfor a white balance process that specifies the light source of thesubject and adjusts colors appropriately.

The image processing circuit 50 is provided with a function to extractonly a frequency component of the signal read from the image pickupdevice 16. The extracted frequency component is used by an automaticfocusing control (AF: Auto Focus). The image processing circuit 50 isprovided with a function to determine which area becomes a target fromwhich the frequency component is extracted among the image data readfrom the image pickup device 16, and a function to perform areadivision. The image processing circuit 50 drives the image pickup device16 in a mode suitable for an AF process when performing the AF process.

The image processing circuit 50 is provided with a function to adjustimage quality of a taken image by operating a signal level, a coloreffect, etc. of the image data digitized by the A/D converter 22. Thereare a function to increase or decrease a signal level with anamplification factor uniform for the whole image, a tone curve (gamma)function to convert the signal level according to intensity of anoriginal signal level, a function to increase or decrease the signallevel with the amplification factor according to the frequency componentfor every area in the image, etc., about the level of the image data.These functions enable level adjustment for various kinds of image data.

The image data digitized by the A/D converter 22 is stored into atemporary memory 30 while it is inputted into the image processingcircuit 50.

The image data appropriately processed by the image processing circuit50 is inputted into an image recognition circuit 38. The imagerecognition circuit 38 recognizes a brightness condition, a focuscondition, and a color condition of the inputted image, and furtherdetects a face area of a person and performs face recognition of thedetected face. When there are characters, the information thereof isrecognized.

In addition to the method of recognizing an image by the imagerecognition circuit 38, the system control unit 60 can also perform theimage recognition process. The system control unit 60 reads the imagedata stored in the temporary memory 30, and analyzes the image data torecognize a condition of a scene.

The system control unit 60 develops the image data processed by theimage processing circuit 50 onto a VRAM 34, converts it into analog datain a D/A convertor 36, and displays it on an image display device 108such as an LCD.

When implementing an electronic viewfinder function, continuous imagedata read from the image pickup device 16 is sequentially displayed onthe image display device 108 and is updated.

The image display device 108 can also display the image data stored inan image storage medium 82. The image storage medium 82 is nonvolatilememory that stores the taken image data via an image-storage-medium I/F(Interface) 80 connected to the data bus 23 and reproduces. A strobeemission circuit 46 controls emission light amount, light-emittingtiming, etc. of a strobe 90 according to instructions from the systemcontrol unit 60. The color temperature of the light emitted by thestrobe unit 90 is preferably stored in a storage unit in the systemcontrol unit 60.

The camera 1 is connectable with external apparatuses 86 such as a PCand a printer via an external-apparatus I/F (Interface) 84.

An operation unit 70 includes a power switch 102, a shutter button 104,a shooting mode selection switch 110, and a parameter selection switch151. The shooting mode selection switch 110 changes camera operationmodes such as a still image shooting mode, a moving image shooting mode,and a reproduction mode. The camera operation modes may include manystill image shooting modes such as a landscape shooting mode, a personshooting mode, etc. that are optimized for a specific scene to be shot.

A user can select shooting conditions at the time of shooting such as afocusing zone, a photometry mode, etc., a page feed at the time ofreproduction of the taken image, the entire settings of the cameraoperations, by means of the parameter selection switch 151. The shutterbutton 104 is one operating member. When depressing the shutter button104 halfway, a photometry switch turns on. When fully depressing theshutter button 104, a release switch turns on.

Next, two examples of persons' faces that are exposed to differentextraneous lights (lights such as sunlight and an indoor illuminationlight other than strobe lights) will be described with reference to FIG.2A and FIG. 2B. FIG. 2A shows a scene in which a person's face isuniformly exposed to the extraneous light and contrast in the person'sface is low. In such a case, it is possible to take a suitable imageregardless of the emission or no-emission of the strobe, when thebrightness of the whole face is measured by average metering, and whenthe aperture, the shutter speed, and the sensitivity are determinedbased on the metering result.

On the other hand, FIG. 2B shows a scene in which a person standsoutdoors under the sunlight in daytime or a person stands directly underan indoor illumination light and contrast in the person's face is high.

When a light source is located over the person's head, while a foreheadpart 311 of the person is exposed to a strong extraneous light, arecessed part 313 around an eye and a mouth-jaw part 315 are not exposedto the extraneous light enough and are shadowed. While a cheek part 321and a nose part 331 are exposed to the strong extraneous light, a lowercheek part 323 and a lower nose part 333 tend to be shadowed.

Thus, when the high contrast occurs in the person's face, a suitablephotograph as portraiture cannot be taken as-is. In such a case, it isdesirable to emit the strobe at the time of the shooting so that theshadowed parts are exposed to the light in order to reduce the contrastin the image.

In the shooting with the strobe emission, when contrast occurs in a facedue to a light source of low color temperature like a filament lampparticularly, it becomes very important what kind of exposure conditionis selected at the time of shooting.

FIG. 3A, FIG. 3B, and FIG. 3C are views showing combination examples ofexposure settings and strobe emissions. Each figure shows a case whenshooting a person whose face has contrast due to a light source LightAof low color temperature. FIG. 3A shows a case where the strobe does notemit. FIG. 3B shows a case where the strobe emits and an exposureunder-compensation process is not executed. FIG. 3C shows a case wherethe strobe emits and the exposure under-compensation process isexecuted.

In the example shown in FIG. 3A, an area 703 near a forehead, a nose,and a cheek of the person is exposed to an illumination light of lowcolor temperature, and an area 707 from a lower cheek to a jaw is hardlyexposed to the illumination light and shadowed.

In the following description, intensities of the illumination light of afilament lamp (LightA) and the strobe light (LightB) will be representedby numeric values corresponding to outputted signal levels from theimage pickup device 16 when the image pickup device 16 receives theselights. In FIG. 3A, the brightness in the area 703 near the forehead,the nose, and the cheek is represented as “LightA=150 LSB (LeastSignificant Bit), LightB=0 LSB”. On the other hand, the brightness ofthe area 707 of the lower cheek is represented as “LightA=6 LSB,LightB=0 LSB”. In this case, since the strobe does not emit, there is noinfluence by the strobe light, and the high contrast occurs by theLightA. Although an AE control is performed in order to achieve correctexposure with respect to the person's face, the contrast cannot bereduced by the AE control only. That is, as a result of performing theAE control so as to achieve the correct exposure by averaging thebrightness in the face including the bright area and the dark area, theface that has no uniformity in the brightness is obtained as a takenimage.

FIG. 3B shows the example when shooting with the strobe emission usingthe exposure condition in FIG. 3A as-is. Here, the example where thestrobe emits with the light amount of 10 LSB is shown. As a result, inFIG. 3B, the brightness of the area 713 near the forehead, the nose, andthe cheek is represented as “LightA=150 LSB, LightB=10 LSB”. On theother hand, the brightness of the area 717 of the lower cheek isrepresented as “LightA=6 LSB, LightB=10 LSB”. This means that the ratioof the light LightB of high color temperature is high in the area 717 ofthe lower cheek, although the ratio of the light LightA of low colortemperature is high in the area 713 near the forehead, the nose, and thecheek. That is, since the color temperature varies with the areas in theface, a suitable taken image is not obtained.

It should be noted that since an actual face has depression andprojection, incident conditions of the strobe light to the respectiveareas are different due to a difference in the distances from the strobeto the areas or a difference in the incident angle of the strobe light.However, in FIG. 3A, FIG. 35, and FIG. 3C, it is described assuming thatthe areas in the face are uniformly exposed to the strobe light of 10LSB in order to simplify the description.

FIG. 3C shows an example when shooting with the strobe emission anddecreasing the exposure by 1.6-step under as compared with the exposurecondition in FIG. 3A. First, the light LightA that exposes the area 723near the forehead, the nose, and the cheek varies from 150 LSB to 50 LSBby setting the exposure at 1.6-step under. On the other hand, the lightLightA that exposes the area 727 from the lower cheek to the jaw variesfrom 6 LSB to 2 LSB. This shows that the exposed light amount of theextraneous light of the low color temperature by the filament lampdecreases.

When emitting the strobe under this exposure condition so that thebrightness of the face becomes proper, the strobe light amount isassumed to be 100 LSB. The brightness in the area 723 near the forehead,the nose, and the cheek is represented “LightA=50 LSB, LightB=100 LSB”.On the other hand, the brightness of the area 727 of the lower cheek isrepresented as “LightA=2 LSB, LightB=100 LSB”. Thus, the light amount ofthe strobe light LightB is more than that of the extraneous light LightAin every area.

Accordingly, the variation in the color temperature of the areas in theperson's face is reduced, it becomes easy to bring the white balance ofthe face close to the proper condition. Also about the contrast in theperson's face, as compared with FIG. 3A and FIG. 3B, the pattern in FIG.3C is improved more, and a more suitable image is obtained.

Thus, it is necessary to correctly detect the contrast in the person'sface that has occurred spontaneously by the extraneous light in order toshoot with the strobe emission so as to reduce the contrast occurred inthe person's face. A phenomenon in which the extraneous light generatesa bright part and a dark part in a person's face occurs according to theconcavo-convex condition of the face. For example, a forehead, a cheek,a nose, etc. are provided as a convex part. An eye, a lower cheek, alower nose, a mouth, etc. are provided as a concave part. Therefore, thecontrast in the face can be detected by metering (brightness detection)a high brightness level in a convex area and a low brightness level in aconcave area.

In this embodiment, a method to acquire brightness distribution bydividing a person's face into ten areas will be described as one ofmethods of detecting contrast in the face.

FIG. 4 is a view showing a person's face divided into ten areas whichare areas used for acquisition of luminance distribution. As a premiseat the time of dividing a face, face information acquired from the facedetection block in the image recognition circuit 38 comprises acoordinate 401 of a face center position in an image, face sizes 403,405, and coordinates 407, 409 of positions of right and left eyes asshown in FIG. 5.

A forehead area shown in FIG. 4 comprises three blocks 413, 415, and417, and is arranged at a position at a predetermined distance from theposition of the person's eye in a direction to the top of head. Anose-cheek area comprises three blocks 423, 425, and 427, and isarranged at a position at a predetermined distance from the position ofthe person's eye in a direction to the jaw. A mouth area comprises threeblocks 423, 425, and 427, and is arranged at a position at apredetermined distance from the position of the person's eye in adirection to the jaw.

Thus, each area is arranged at the position at the predetermineddistance from the position of the eye. Here, a value corresponding tothe face sizes 403 and 405 is set to the predetermined distance as apredetermined ratio with respect to the face sizes.

In this embodiment, even if the positions of the parts other than theeye in the face are unknown, the areas can be set up. However, when theface information acquired from the face detection block in the imagerecognition circuit 38 includes varieties of information about a nose, amouth, and a cheek, etc., the areas may be set using them.

It should be noted that the blocks 423, 425, and 427 in the nose-cheekarea and the blocks 433, 435, and 437 in the mouth area are set up so asnot to overlap. And finally an area 441 is set up to cover the wholeface.

Thus, the face is divided into the ten areas and a histogram ofbrightness is acquired from each area. FIG. 6A and FIG. 6B are the viewsshowing the areas in the face acquired as information about a histogram.When acquiring a histogram, as shown in FIG. 6A, three pieces ofbrightness distribution information about the three blocks 413, 415, and417 in the forehead area are combined, and information (data) about onehistogram in the forehead area 411 is acquired. In the same manner,three pieces of brightness distribution information about the threeblocks 423, 425, and 427 in the nose-cheek area are combined, andinformation (data) about one histogram in the nose-cheek area 421 isacquired.

As shown in FIG. 6B, six pieces of brightness distribution informationabout the six blocks 423, 425, 427, 433, 435, and 437 in anose-cheek-mouth area are combined, and information (data) about onehistogram in the nose-cheek-mouth area 431 is acquired.

Thus, since the areas are beforehand divided into small areas,respectively, the brightness distribution corresponding to a complexfacial shape can be acquired by adding the brightness distributions ofthese small areas when acquiring the brightness distribution. Thehistogram in the forehead area 411 and the histogram in the nose-cheekarea 421 that are acquired here are used for detecting a high brightnessarea in a person's face. On the other hand, the histogram in thenose-cheek-mouth area 431 is used for detecting a low brightness area.

Thus, since the person's face is divided into areas, it becomes possibleto acquire the person's forehead area histogram, the nose-cheek areahistogram, the nose-cheek-mouth area histogram, and whole face histogramas a histogram of brightness.

FIG. 7A, FIG. 7B, FIG. 7C, and FIG. 7D are examples of histogramsshowing brightness distributions of the respective areas. In thesehistograms that comprises the forehead area histogram, the nose-cheekarea histogram, the nose-cheek-mouth area histogram, and the whole facehistogram, a horizontal axis represents the brightness value (8 bits),and a vertical axis represents the number of pixels (pixel count) havingthe specific brightness value shown by the horizontal axis.

FIG. 7A and FIG. 7B show the forehead area histogram and the nose-cheekarea histogram that are equivalent to the areas 411 and 421 in FIG. 6A,respectively. In both of the forehead area histogram and the nose-cheekarea histogram, since the pixel count increases as the brightnessbecomes higher, it can be assumed that there is no forelock or hat thathides the forehead of the person, and that the person is exposed to astrong extraneous light. When black forelock etc. hides the forehead, adistribution inclined toward the high brightness side like FIG. 7A isnot seen. When the face is not exposed to the strong extraneous light,the distribution inclined toward the high brightness side is not seen ineither the forehead area histogram or the nose-cheek area histogram.

In order to grasp how many pixels are distributed to the high brightnessside, a brightness value LightY(n) at a position of a specific ratio toa distributed total pixel number is calculated. The brightness valueLightY(n) represents the brightness value of the pixel of which thecount is no of the distributed total pixel number when counting from thepixel of the least brightness value in the histogram. For example, inthe forehead area histogram in FIG. 7A, the brightness values 617, 615,613, and 611 are calculated as the brightness values LightY(70),LightY(85), LightY(90), and LightY(95) for the four specific ratios atthe high brightness side.

Similarly, in the nose-cheek area histogram in FIG. 75, the brightnessvalues 627, 625, 623, and 621 are calculated as the brightness valuesLightY(70), LightY(85), LightY(90), and LightY(95) for the four specificratios at the high brightness side. It should be noted that a brightnessvalue averaging the brightness values 627, 625, 623, and 621, or thebrightness value of the highest pixel number can be used as ahigh-brightness value LightY mentioned later.

When the forehead is hidden, it is considered that the brightness valueLightY(n) of the forehead area is low. In this case, it is preferable tocompare the brightness value LightY(n) calculated from the forehead areawith the brightness value LightY(n) calculated from the nose-cheek area,and to use the higher brightness value LightY(n) as the high-brightnessvalue.

FIG. 7C shows a nose-cheek-mouth area histogram corresponding to thearea 431 in FIG. 63. In this embodiment, it is assumed that the facedetection block in the image recognition circuit 38 cannot detect avertex position of a convex-part of the cheek. Therefore, a histogramincluding blocks 423, 425, and 427 in the cheek area is acquired.Accordingly, a certain number of pixels are distributed to the highbrightness side.

Since the nose-cheek-mouth area histogram is used to detect a shadow ofa low brightness occurred in the person's face, it is hardly affectedwhen a certain number of pixels are distributed to the high brightnessside. Therefore, the histogram is used to calculate a brightness valueShadeY(n) at a position of a specific ratio at the low brightness side.The brightness value ShadeY(n) represents the brightness value of thepixel of which the count is n % of the distributed total pixel numberwhen counting from the pixel of the least brightness value in thehistogram. And the brightness values 631, 633, 635, and 637 arecalculated as the brightness values ShadeY(5), ShadeY(10), ShadeY(15),and ShadeY(30) for the four specific ratios like the high brightnessside. It should be noted that a brightness value averaging thebrightness values 631, 633, 635, and 637, or the brightness value of thehighest pixel number can be used as a low-brightness value ShadeYmentioned later.

FIG. 7D shows the whole face histogram corresponding to the area 441 inFIG. 4 that include the main positions of a face such as a forehead, aneye, a cheek, a nose, and a mouth. The average brightness Y of a face iscalculated using this whole face histogram. The brightness value 641 ofthe pixel of which the count is 50% of the distributed total pixelnumber is calculated as a brightness value FaceY.

Next, a method of determining an exposure under-compensation valuecorresponding to color temperature of an extraneous light, facecontrast, and subject brightness will be described according to aflowchart shown in FIG. 8. FIG. 8 is the flowchart showing an exposurecompensation value calculation procedure. This processing program isstored in a storage medium in the system control unit 60, and isexecuted by a CPU in the system control unit 60.

First, the system control unit 60 collects the information needed forcalculating an exposure compensation value (step S31). Specifically, thesystem control unit 60 collects the brightness value LightY asinformation about the brightness in the high brightness area of the faceregion acquired by the above-mentioned histogram analysis process, thebrightness value ShadeY as information about the brightness in the lowbrightness area of the face region, the color temperature Temp asinformation about the color temperature in the image, and the subjectbrightness Bv.

As the color temperature Temp in the image, the value acquired as a partof an AWB process is used. An origin of this color temperature Temp isthe color information extracted by the image processing circuit 50. Thesubject brightness By as information about the brightness of the subjectis acquired as a part of an AE process. An origin of this subjectbrightness Bv is the brightness information extracted by the imageprocessing circuit 50. Not only the brightness of the person whose facecontrast has calculated, but also the brightness of the other area inthe taken image is considered. At this time, the subject brightness Bvmay be calculated by averaging the brightness values acquired at aplurality of areas in the taken image or may be acquired by adding thebrightness values that have been acquired at a plurality of areas in thetaken image and modified by predetermined weights.

The AE process controls the exposure so that average brightness within aface of a target person becomes proper brightness, and similarlycontrols the exposure when there are variations of light and shade inthe face. As a result, there are the brightness value LightY of a brightpart and the brightness value ShadeY of a dark part with respect to theproper brightness. The system control unit 60 calculates an exposureunder-compensation value that compensates for the brightness valueLightY of the bright part in the face to be proper brightness accordingto the following equation (1) (step S32). Here, RefY is targetbrightness for controlling to be proper exposure with respect to theface area while executing the AE process.

$\begin{matrix}{{ExpCompLight} = {{Log}_{2}\frac{{Ref}\; Y}{{Light}\; Y}}} & (1)\end{matrix}$

When the exposure value calculated by the AE process is corrected inunderexposure by the above-mentioned ExpCompLight value, there is nopart brighter than the proper brightness in the face.

The system control unit 60 calculates an ExpCompTemp value as anexposure under-compensation value (first compensation value)corresponding to a color temperature difference between the strobe lightand the extraneous light (step S33). A predetermined graph shown in FIG.9 is used for this calculation. FIG. 9 is the graph showing correlationbetween the target brightness corresponding to the color temperature ofthe extraneous light and an under-compensation step. A horizontal axisrepresents the color temperature of the extraneous light in a rangebetween 2500 K and 5500 K, and also represents the color temperaturedifference between the strobe light and the extraneous light in a rangebetween 0 K and 3000 K. A vertical axis represents the target brightnessused when controlling by the AE process to be correct exposure, and theunder-compensation step that compensates it.

The graph in FIG. 9 assumes that the camera is provided with the strobethat emits a light having the color temperature of 5500 K as shown by astrobe light color temperature 801, and that the target brightness levelof the AE process is 180 LSB as shown by face proper brightness 803.

A point 811 in FIG. 9 means that the target brightness keeps 180 LSBwithout the under-compensation and the compensation step is ±0, whenthere is no color temperature difference between the strobe light andthe extraneous light.

As compared with this, a point 813 means that the color temperaturedifference between the strobe light and the extraneous light becomes−3000 K because the extraneous light has low color temperature of 2500K, the target brightness becomes 90 LSB as a result of theunder-compensation, and the compensation step is −1.0.

Thus, since the target brightness decreases as the color temperaturedifference between the strobe light and the extraneous light increases,the exposure is set so that the extraneous light amount that impinges onthe person's face decreases as the color temperature differenceincreases. The target brightness under-compensations value calculatedusing the table in FIG. 9 and the color temperature difference becomesthe ExpCompTemp value that is the exposure under-compensation valuecorresponding to the color temperature of the extraneous light.

Next, the system control unit 60 calculates an ExpCompContrast value asan exposure under-compensation value (second compensation value)corresponding to the face contrast level (step S34). That is, theexposure under-compensation value is calculated based on a comparisonresult of the information about the brightness in the high brightnessarea in the face area with the information about the brightness in thelow brightness area. A predetermined graph shown in FIG. 10 is used forthis calculation. FIG. 10 is the graph showing correlation between thetarget brightness corresponding to the face contrast level and theunder-compensation step. A horizontal axis represents a step showing theface contrast level. The face contrast level represents brightnessdifference between the high-brightness part (area) in the face and thelow-brightness part (area), and is expressed as a value of ΔContrastthat is calculated according to the following equation (2) using thebrightness value LightY and the brightness value ShadeY that arecalculated from the histogram analysis of FIG. 7A, FIG. 7B, and FIG. 7C.

$\begin{matrix}{{\Delta \; {Contrast}} = {{Log}_{2}\frac{{Light}\; {Y(1)}}{{Shade}\; {Y(1)}}}} & (2)\end{matrix}$

A vertical axis represents the target brightness used when controllingby the AE process to be correct exposure, and the under-compensationstep that compensates it. In FIG. 10, the face contrast level ±0.0 is astep 901 when there is no face contrast. The graph in FIG. 10 shows acondition where there is no high contrast between the convex parts (suchas a forehead, a nose, and a cheek) and the concave parts (from a lowercheek to a jaw).

The point 911 shows that the target brightness keeps 180 LSB and thecompensation step is 0 because the face contrast level is 0. On theother hand, the point 913 shows that the face contrast level between thehigh brightness part and the low brightness part in the face is +4, andin this case, the target brightness becomes 90 LSB as a result of theunder-compensation, and the compensation step becomes −1.0.

Thus, the exposure is set so that the target brightness decreases as theface contrast level increases (as the brightness difference between thehigh brightness part and the low brightness part becomes large). Theexposure under-compensation value calculated using the graph in FIG. 10and the face contrast level ΔContrast becomes an ExpCompContrast valuethat is an exposure under-compensation value according to the facecontrast level.

According to the earlier mentioned process, the two compensation values,which includes the ExpCompTemp value that is an exposureunder-compensation value corresponding to the color temperaturedifference and the ExpCompContrast value that is an exposureunder-compensation value corresponding to the face contrast level, arecalculated. And then, the process proceeds to a next step S35.

The system control unit 60 determines whether the ExpCompTemp value thatis an exposure under-compensation value corresponding to the colortemperature difference is smaller than the ExpCompContrast value that isan exposure under-compensation value corresponding to the face contrastlevel, i.e., whether the compensation for the color temperaturedifference needs larger amount of compensation (step S35).

When the ExpCompTemp value is smaller than the ExpCompContrast value,and needs larger amount of compensation, the system control unit 60executes the following process. That is, the system control unit 60clips the ExpCompTemp value by the ExpCompContrast value (step S36).

This process decreases the exposure under-compensation amount withrespect to the target brightness (increases the compensation value) whenthe color temperature difference between the strobe light and theextraneous light is large but when the face contrast level is small.That's because the color temperature difference in the face by shootingwith the strobe emission is small without respect to the strobe lightamount even if the color temperature difference between the strobe lightand the extraneous light is large when the face contrast level is small.In this embodiment, the above-mentioned two correlation graphs shown inFIG. 9 and FIG. 10 are provided, the exposure under-compensation valuesare found from the respective graphs, and the exposureunder-compensation value is determined by a matrix of both the values.

As another method, the exposure under-compensation value may becalculated according to a predetermined formula (a functionalexpression) rather than the correlation graphs.

When it is determined that the ExpCompTemp value is equal to or largerthan the ExpCompContrast value in the step S35, or when the process inthe step S36 is executed, the system control unit 60 executes thefollowing process. That is, the system control unit 60 calculates anexposure under-compensation value ExpComp by summing the exposureunder-compensation value ExpCompLight and the exposureunder-compensation value ExpCompTemp (step S37). The exposureunder-compensation value ExpCompLight is a value for adjusting thebrightness value LightY of the bright parts in the face calculated inthe step S33 with the target brightness. On the other hand, the exposureunder-compensation value ExpCompTemp is obtained from the targetbrightness determined by the color temperature difference and the facecontrast level.

In steps S38, S39, and S40, the system control unit 60 executes aclipping process corresponding to the subject brightness Bv with respectto the exposure under-compensation value ExpComp calculated in the stepS37. This clipping process prevents executing the exposureunder-compensation process too much in a high-brightness environmentlike outdoors in daytime. The exposure under-compensation process isexecuted depending on the color temperature of the extraneous light andthe face contrast. When executing the exposure under-compensationprocess in the high-brightness environment like the outdoors in daytime,a background which is not exposed to the strobe light may become darkeven when shooting in a bright location. Therefore, the clipping processmentioned above is executed so as to acquire a more suitable image inthe high-brightness environment like the outdoors in daytime.

As mentioned above, a strobe emission is effective in the followingsituation. That is, when there is high possibility to cause subjectmoving and hand shake due to slow shutter speed in a low illuminationenvironment, the shake can be prevented by increasing shutter speed andunder-exposure can be compensated with a strobe light. In this case,since the shutter speed increases, the whole image is in underexposure,a background which is not exposed to the strobe light holds dark.Although the phenomenon in which a background becomes dark when shootingwith the strobe emission has a small influence in a low illuminationenvironment, it has a large influence in a high brightness environmentlike the outdoors in daytime.

The system control unit 60 calculates an exposure under-compensationlimit value LimExpComp compensation limit value) corresponding to thesubject brightness Bv according to the graph in FIG. 11 (step S38). FIG.11 is the graph showing the exposure under-compensation limit valuecorresponding to the subject brightness. A horizontal axis representsthe subject brightness Bv. On the other hand, a vertical axis representsthe exposure under-compensation limit value.

At a point 1001, the subject brightness is as high as Bv=8. In such ahigh brightness case, since there is a high possibility to be in theoutdoors in daytime, the exposure under-compensation limit value isclipped as 0.0 step not to execute the exposure under-compensationprocess.

At a point 1013, the subject brightness is as low as Bv=4. In such a lowbrightness case, there is a high possibility to be indoors and theexposure under-compensation limit value is set as −2.5 steps. Thus, thepossibility to be in the indoor (i.e., the possibility not to be in theoutdoors in daytime) becomes higher as the brightness decreases, and itcan be determined that the influence becomes smaller even if thebackground becomes dark.

The system control unit 60 compares the exposure under-compensationlimit value LimExpComp, which is calculated corresponding to the subjectbrightness Bv according to the graph in FIG. 11, with the exposureunder-compensation value ExpComp calculated in the step S37 (step S39).When the exposure under-compensation value ExpComp is smaller than theexposure under-compensation limit value LimExpComp (i.e., thecompensation amount is larger), the system control unit 60 clips theexposure under-compensation value ExpComp (step S40). Thus, a settingrange of the exposure under-compensation value ExpComp is restricted.Then, the system control unit 60 finishes the process.

On the other hand, when it is determined that the exposureunder-compensation value ExpComp is equal to or larger than the exposureunder-compensation limit value LimExpComp at step S39, the systemcontrol unit 60 finishes this process as-is.

Thus, the exposure under-compensation value ExpComp that is the exposureunder-compensation value with respect to the target brightness inconsideration of the color temperature difference between the strobelight and the extraneous light, the face contrast level, and the subjectbrightness is calculated by the process shown in FIG. 8.

FIG. 12 is a flowchart showing the entire operation when shooting by acamera having the function of the exposure under-compensation processshown in FIG. 8. This processing program is stored in a storage mediumin the system control unit 60, and is executed by the CPU in the systemcontrol unit 60.

After starting a camera, the system control unit 60 supplies electricpower to units in the camera, and executes a system startup process forthe CPU, a memory, etc. (step S1).

Then, the system control unit 60 starts various devices such as azooming motor for moving the lens 10 mounted on the camera to a shootingposition, a focusing motor, the diaphragm 14, and the mechanical shutter12 (step S2).

After starting the system and devices that are needed to operate thecamera, the system control unit 60 starts AE/AF/AWB processes in orderto display a subject image on an LCD under a suitable condition inbrightness, focus, and colors (step S3).

When the preparation to display a subject image in the suitablecondition is completed, the system control unit 60 displays thecontinuous image (live image) read from the image pickup device 16 ontothe image display device 108 (step S4).

During the display of the live image, the system control unit 60periodically repeats the AE, AF, and AWB processes to continuouslymaintain the suitable condition in brightness, focus, and colors even ifan operation is inputted by a photographer or the subject itself changes(steps S5, S6, and S7).

The face contrast level is calculated by another routine executed inparallel to the above-mentioned process. The system control unit 60acquires a target person's face contrast level calculated by thisface-contrast-level calculation process (step S8).

The system control unit 60 monitors whether the photometry switch turnedon by depressing the shutter button 104 halfway (step S9). When thephotometry switch does not turn on, the system control unit 60 returnsthe process to the step S5. On the other hand, when the photometryswitch turns on, the system control unit 60 executes variouspre-processes accompanying shooting.

First, the system control unit 60 executes the AE process that setsshooting exposure (i.e., an aperture, a shutter speed, and asensitivity) properly corresponding to brightness of a subject (stepS10). At this time, the exposure is determined with the assumption thatthe strobe does not emit. Then, the system control unit 60 executes theAF process to bring the subject into focus (step S11). Here, the systemcontrol unit 60 does not only bring the subject into focus but alsocalculates a distance from the camera to the subject.

After the exposure is determined in the step S10 and the focus isdetermined in the step S11, the system control unit 60 executes a strobeemission determination process to determine whether the strobe will beemitted (step S12). In this strobe emission determination, variousfactors are considered.

For example, since a possibility to shake increases when the shutterspeed determined by the exposure determination in the step S10 is lessthan a predetermined value, the shutter speed is reset higher and astrobe emission is determined.

In this embodiment, the face-contrast computed result acquired in thestep S8 is referred at the time of this strobe emission determination,and a strobe emission is determined when a face has contrast.

The system control unit 60 re-determines the shooting exposure only whenemitting the strobe. That is, the system control unit 60 executes acompensation process of the exposure determined in the step 210 forevery factor of the strobe emission (step S13).

For example, when the strobe emission is determined to reduce shake dueto a slow shutter speed, the system control unit 60 compensates theshutter speed to increase by steps to prevent the shake. When the strobeemission is determined due to the face contrast in this embodiment, thesystem control unit 60 compensates the exposure in an under direction bythe exposure under-compensation value ExpComp determined by the processshown in FIG. 8.

When the shooting conditions are fixed by turning on the photometryswitch, the system control unit 60 monitors whether the release switchis turned on as a shooting instruction (step S14). When the photometryswitch turns off without turning on the release switch, the systemcontrol unit 60 returns the process to the step S5.

On the other hand, when detecting the release switch turns on, thesystem control unit 60 determines whether it emits the strobe, based onthe result of the strobe emission determination (step S15). When thestrobe emission has been determined in the step S12, the system controlunit 60 executes a process that determines strobe light amount at thetime of shooting according to this determination (step S16).

This process is known as a metering process to obtain a reflectance anda size of a subject based on subject brightness exposed to only anextraneous light and subject brightness exposed to a pre-emission strobelight of a predetermined light amount, and to calculate light amount ofthe strobe light to be emitted at the time of shooting. In this case,the light amount is calculated in consideration of the exposurecompensation amount (the exposure under-compensation value) ExpCompcalculated by the process in FIG. 9. That is, the emission light amountof the strobe increases so as to compensate the exposure reduced by theexposure compensation. When shooting a person's face, fundamentally, thebrightness of the face is obtained by averaging the brightness values ofthe respective parts in the face, and the emission light amount isdetermined so that the average brightness becomes the proper brightness.

After determining the emission light amount of the strobe, the systemcontrol unit 60 executes the shooting process (step S17). Then, thesystem control unit 60 finishes the process. If the image pickup deviceis exposed to the suitable light and a development process is performed,an image with preferred brightness, focus, and color is obtained.

In this development process, the brightness of a low brightness area inthe image can be corrected to be brighter. For example, when there isface contrast, the contrast level is reduced by applying the exposureunder-compensation process and then shooting with the strobe emission.However, a dark part may remain in the face due to an unevendistribution of the strobe light etc. In such a case, a more suitableimage can be obtained by increasing brightness of the dark part in theface during the development process.

Thus, the image pickup apparatus according to the embodiment reduces thedifference in the colors of lights illuminating the respective parts inthe face at the time of shooting with the strobe emission by executingthe exposure under-compensation process to decrease the extraneouslight, when the face has contrast due to difference in color temperaturebetween the strobe light and the extraneous light. Simultaneously, theapparatus is possible to obtain a suitable image by reducing thecontrast difference in the face.

That is, the camera detects that the person's face has brightnessdifference due to the extraneous light having different colortemperature from the strobe light. When emitting the strobe in such acondition, the camera controls the strobe emission by adjusting exposureby decreasing a ratio of the extraneous light amount to the strobe lightamount so as to reduce the difference in color temperatures of the parts(convex parts and concave parts) in the face. Accordingly, thebrightness difference between the high brightness area and the lowbrightness area in the face can be reduced, and a strobe emission imagewith a suitable face color can be obtained because the color temperaturedifference of each part of the person's face decreases. At this time,when considering not only the person's face color, but also backgroundbrightness to maintain suitable brightness corresponding to subjectbrightness, more preferable image can be shot.

It should be noted that the present invention is applicable to not onlythe configuration of the above-mentioned embodiment, but also anyconfigurations as long as the functions shown in the claims or thefunctions of the configuration of the embodiment can be achieved.

For example, although the above-mentioned embodiment describes the casewhere a person's face is shot as a subject, the present invention is notlimited to this and can be applied to the case where various subjectssuch as animals and ornaments are shot.

In the above-mentioned embodiment, the high brightness area is a convexpart in a face, and the low brightness area is a concave part in a face.However, an area having high brightness may serve as the high brightnessarea, and an area having low brightness may serve as the low brightnessarea, without respect to concave and convex parts in a subject.

Although the above-mentioned embodiment describes the camera withbuilt-in strobe as a light emitting unit as an example, the presentinvention can be applied to a camera with a detachable strobe.

In the above-mentioned embodiment, although the digital camera ofcompact type has been described as an example of the image pickupapparatus, the present invention is also applicable to a digital camerahaving an exchangeable lens unit, a video camera, etc.

In the above-mentioned embodiment, although the exposure compensationamount is determined in consideration of subject brightness, theexposure compensation amount may be determined without taking thesubject brightness into consideration. For example, when a shooting modeoptimized to indoor shooting (a party mode etc.) is selected from amongshooting modes optimized to specific shooting scenes, the exposurecompensation amount may be determined without taking the subjectbrightness into consideration.

Other Embodiments

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiment(s), and by a method, the steps ofwhich are performed by a computer of a system or apparatus by, forexample, reading out and executing a program recorded on a memory deviceto perform the functions of the above-described embodiment(s). For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (e.g., computer-readable medium).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2009-289343, filed on Dec. 21, 2009, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image pickup apparatus that is possible toshoot with emitting light by a light emitting unit, a color temperatureof the emitted light by the light emitting unit being fixed, theapparatus comprising: a color information acquisition unit configured toacquire color information related to a light illuminating a subject; andan emission control unit configured to control emission of the lightemitting unit, wherein the emission control unit determines emissionlight amount of the light emitting unit based on a difference betweenpredetermined color information and color information acquired in a casewhere the light emitting unit does not emit the light by the colorinformation acquisition unit.
 2. The image pickup apparatus according toclaim 1, wherein the predetermined color information is related to thecolor temperature of the emitted light by the light emitting unit. 3.The image pickup apparatus according to claim 1, wherein the emissioncontrol unit increases the emission light amount as the differenceincreases.
 4. The image pickup apparatus according to claim 1, furthercomprising: a brightness information acquisition unit configured toacquire information about the brightness of the subject, wherein theemission control unit determines emission light amount of the lightemitting unit based on the difference, and a comparison result betweenbrightness information in a high brightness area in the subject andbrightness information in a low brightness area in the subject that areacquired by the brightness information acquisition unit.
 5. The imagepickup apparatus according to claim 3, wherein the emission control unitincreases the emission light amount as difference between the brightnessinformation in the high brightness area and the brightness informationin the low brightness area based on the information acquired by thebrightness information acquisition unit increases.
 6. The image pickupapparatus according to claim 1, further comprising: an exposure controlunit configured to control exposure, wherein the exposure control unitcontrols the exposure at the time of shooting with emitting light by thelight emitting unit based on the difference.
 7. The image pickupapparatus according to claim 6, wherein the exposure control unit shiftsthe exposure to underexposure as the difference increases.
 8. The imagepickup apparatus according to claim 7, wherein the emission control unitdetermines the emission light amount based on the sifted exposure.
 9. Animage pickup apparatus that is possible to shoot with emitting light bya light emitting unit, the apparatus comprising: a color informationacquisition unit configured to acquire color information related to alight illuminating a subject; an exposure control unit configured tocontrol exposure; and an emission control unit configured to controlemission of the light emitting unit, wherein the exposure control unitdetermines a exposure value at the time of shooting with emitting lightby the light emitting unit based on a difference between predeterminedcolor information and color information acquired in a case where thelight emitting unit does not emit the light by the color informationacquisition unit, wherein the emission control unit determines emissionlight amount of the light emitting unit based on the exposure value. 10.An image pickup apparatus that is possible to shoot with emitting lightby a light emitting unit, the apparatus comprising: a color informationacquisition unit configured to acquire color information related to alight illuminating a subject; an exposure control unit configured tocontrol exposure; and an emission control unit configured to controlemission of the light emitting unit, wherein the exposure control unitdetermines a exposure value at the time of shooting with emitting lightby the light emitting unit based on a difference between predeterminedcolor information and color information acquired in a case where thelight emitting unit does not emit the light by the color informationacquisition unit, wherein the emission control unit determines emissionlight amount of the light emitting unit based on the difference.
 11. Acontrol method for an image pickup apparatus is possible to shoot withemitting light by a light emitting unit, a color temperature of theemitted light by the light emitting unit being fixed, the methodcomprising: a color information acquisition step of acquiring colorinformation related to a light illuminating a subject; and an emissioncontrol step of controlling emission of the light emitting unit, whereinthe emission light amount of the light emitting unit is determined inthe emission control step based on a difference between predeterminedcolor information and color information acquired in a case where thelight emitting unit does not emit the light in the color informationacquisition step.
 12. A control method for an image pickup apparatusthat is possible to shoot with emitting light by a light emitting unit,the method comprising: a color information acquisition step of acquiringcolor information related to a light illuminating a subject; an exposurecontrol step of controlling exposure; and an emission control step ofcontrolling emission of the light emitting unit, wherein a exposurevalue is determined in the exposure control step at the time of shootingwith emitting light by the light emitting unit based on a differencebetween predetermined color information and color information acquiredin a case where the light emitting unit does not emit the light in thecolor information acquisition step, wherein emission light amount of thelight emitting unit is determined in the emission control step based onthe exposure value.
 13. A control method for an image pickup apparatusthat is possible to shoot with emitting light by a light emitting unit,the method comprising: a color information acquisition step of acquiringcolor information related to a light illuminating a subject; an exposurecontrol step of controlling exposure; and an emission control step ofcontrolling emission of the light emitting unit, wherein a exposurevalue is determined in the exposure control step at the time of shootingwith emitting light by the light emitting unit based on a differencebetween predetermined color information and color information acquiredin a case where the light emitting unit does not emit the light in thecolor information acquisition step, wherein emission light amount of thelight emitting unit is determined in the emission control step based onthe difference.